U.S. patent number 4,610,062 [Application Number 06/647,957] was granted by the patent office on 1986-09-09 for method of making an acoustic microphone.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to Thomas E. Hendrickson, Jon A. Roberts.
United States Patent |
4,610,062 |
Roberts , et al. |
September 9, 1986 |
**Please see images for:
( Certificate of Correction ) ** |
Method of making an acoustic microphone
Abstract
A capacitive acoustic transducer comprises a first field plate
mounted to a semiconductor substrate structure, a second metal
field plate having first and second oppositely and substantially
exposed surfaces, and a mounting arrangement for mounting the
second field plate to the semiconductor substrate structure so that
the first and second field plates form an acoustically responsive
capacitor, the first exposed surface of the second field plate
substantially facing the first field plate, the mounting
arrangement allowing the second field plate to respond to acoustic
energy for altering the capacitance between the first and second
field plates, the mounting arrangement including first and second
contacts for respective connection to the first and second field
plates. The method of fabricating this capacitive acoustic
transducer includes the step of forming the second field plate on a
disposable support structure, mounting the second field plate on
the disposable support structure to the first field plate on the
substrate structure and then disposing of the disposable support
structure.
Inventors: |
Roberts; Jon A. (Minnetonka,
MN), Hendrickson; Thomas E. (Wayzata, MN) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
|
Family
ID: |
27034569 |
Appl.
No.: |
06/647,957 |
Filed: |
September 6, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
446290 |
Dec 2, 1982 |
4495385 |
|
|
|
Current U.S.
Class: |
29/25.41;
29/423 |
Current CPC
Class: |
G01L
9/0072 (20130101); H04R 19/04 (20130101); Y10T
29/43 (20150115); Y10T 29/4981 (20150115) |
Current International
Class: |
G01L
9/00 (20060101); H04R 19/04 (20060101); H04R
19/00 (20060101); H01G 007/00 () |
Field of
Search: |
;29/25.42,25.41,423,424
;179/111R,111E,121R ;228/180.2 ;361/328 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; Howard N.
Assistant Examiner: Echols; P. W.
Attorney, Agent or Firm: Joike; Trevor B.
Parent Case Text
This application is a division of application Ser. No. 446,290,
filed 12/2/82 now U.S. Pat. No. 4,495,385.
Claims
The embodiments of the invention in which an exclusive property or
right is claimed are defined as follows:
1. A method for fabricating a capacitive acoustic transducer
comprising:
forming a first field plate on a substrate structure;
forming a second field plate on a disposable support structure;
providing first and second contact means for respective connection
to said first and second field plates;
mounting said second field plate and disposable support structure
to said substrate structure so that said first and second field
plates form an acoustically responsive capacitor; and,
disposing of said disposable support structure.
2. The method of claim 1 wherein said step of forming a second
field plate on a disposable support structure comprises the step of
forming said second field plate on a soluble salt.
3. The method of claim 2 wherein said step of forming a second
field plate on a disposable support structure comprises the step of
forming a second field plate having a non-solder wettable
portion.
4. The method of claim 3 wherein said step of forming a second
field plate having at least a non-solder wettable portion on a
disposable support structure comprises the step of forming a second
field plate having a portion which is solder wettable.
5. The method of claim 4 wherein said step of providing first and
second contact means comprises the step of providing a solder bump
on the solder wettable portion of said second field plate.
6. The method of claim 5 wherein said step of mounting said second
field plate and said disposable support structure to said substrate
structure comprises the step of providing stand-offs between said
first and second field plates.
7. The method of claim 6 wherein said mounting step comprises the
step of flowing the solder and stretching the second field
plate.
8. The method of claim 7 wherein said step of providing first and
second contact means comprises the step of providing at least one
solder bump to said second field plate.
9. The method of claim 8 wherein said mounting step comprises the
step of flowing said solder and stretching said diaphragm.
10. The method of claim 9 wherein said step of forming a second
field plate on a disposable support structure comprises the step of
forming a second field plate having a solder wettable portion and a
solder non-wettable portion and wherein said solder bump is applied
to said solder wettable portion.
Description
BACKGROUND OF THE INVENTION
The present invention relates to capacitive acoustic microphones
and, more particularly, to capacitive acoustic microphones
involving a first field plate mounted to a semiconductor substrate
structure and a second metal field plate mounted to the
semiconductor substrate structure so that the first and second
field plates form an acoustically responsive capacitor. The
invention also involves the method of fabricating such a
capacitor.
Various capacitive microphones have been known in the prior art.
Typically, these microphones are either constructed from discrete,
non-semiconductor materials which do not lend themselves to
semiconductor batch fabrication or constructed by use of
semiconductor batch fabrication technology but without the
advantages that nontraditional semiconductor fabrication can
provide.
In the former case, the diaphragm electrode of the capacitive
transducer is mechanically stretched over a void created in a base
plate which is typically an insulative base plate such as alumina.
A second electrode is then plated to the insulative base plate to
form the second plate of the capacitor. The process of making such
a transducer is time consuming and cannot take advantage of
semiconductor processing which can, as an example, fabricate both
the transducer and the electronic signal processing circuitry on
the same semiconductor substrate.
In the latter case, where semiconductor batch fabrication
techniques are used for making the transducer, both the diaphragm
electrode and the stationary fixed electrode are constructed from
semiconductor materials. Such arrangements are expensive to
construct and do not yield optimum capacitive type transducers.
SUMMARY OF THE INVENTION
The present invention provides a low cost, moderate performance,
low power microphone including a first field plate mounted to a
semiconductor substrate structure, a second metal field plate
having first and second oppositely and substantially exposed
surfaces, and a mounting arrangement for mounting the second field
plate to the semiconductor substrate structure so that the first
and second field plates form an acoustically responsive capacitor,
the first exposed surface of the second field plate substantially
facing the first field plate, the mounting arrangement allowing the
second field plate to respond to acoustic energy for altering the
capacitance between the first and second field plates, the mounting
arrangement also including first and second contacts for respective
connection to the first and second field plates.
The present invention also involves a method of fabricating such a
capacitive acoustic microphone including the steps of forming a
first field plate on a substrate structure, forming a second field
plate on a disposable support structure, providing first and second
contacts for respective connection to the first and second field
plates, mounting the second field plate and disposable support
structure to the substrate structure so that the first and second
field plates form an acoustically responsive capacitor, and
disposing of the disposable support structure.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages will become more apparent
from a detailed consideration of the invention when taken in
conjunction with the drawings in which:
FIGS. 1-4 show the construction of the first field plate mounted on
a semiconductor substrate;
FIGS. 5-7 show the construction of the second field plate forming
the diaphragm of the capacitive acoustic microphone;
FIGS. 8 and 9 show the two field plate constructions joined;
and,
FIGS. 10 and 11 show the capacitive acoustic transducer in more
detail.
DETAILED DESCRIPTION
FIGS. 1-9 show the fabrication steps in making a capacitive
acoustic transducer according to the present invention.
The transducer shown in FIGS. 1-9 is simplified for explanatory
purposes and is shown in more detail in FIGS. 10 and 11 as will be
discussed hereinafter. The fabrication sequence as represented by
the drawings may, for example, begin after completion of the
process steps which integrate CMOS electronic signal processing
circuitry on a semiconductor substrate such as substrate 11 shown
in FIG. 1. This substrate may be p type silicon, for example. A
first field plate 12 is formed on or in substrate 11. Lower field
plate 12 can be a metal plate, for example, plated, evaporated or
sputtered onto substrate 11 or may be diffused into substrate 11 by
any known diffusion process so that first field plate 12 may, for
example, be an n+ layer in substrate 11. Lower field plate 12 could
also be a polysilicon or other deposited conductive layer.
A dielectric passivation layer such as silicon dioxide, SiO.sub.2,
is typically formed over substrate 11 but has been omitted to
simplify the explanation of the process. Additionally, passivation
layer 13, which can be Si.sub.3 N.sub.4, may also be provided for
environmental protection of the CMOS circuitry. This passivation
layer 13 may not be essential depending upon the environment in
which the transducer is used.
At this point, the wafer, if batch fabrication techniques are used,
has contact pads 14, which may be platinum, formed thereon. Then,
as shown in FIG. 3, a thick (1-5 mils) layer of dielectric is
deposited and patterned to form circular stand-off rings 15. The
stand-off rings are designed to provide added support for the
diaphragm when it is mounted to this unit. A top view of the
transducer portion of the wafer is shown in FIG. 4.
As shown in FIG. 5, the diaphragm is formed by depositing a thin
layer (for example 0.2-2 micrometers) of metal 16 (such as
titanium) on a disposable support or substrate 17. This disposable
support may be a soluble substrate such as calcium chloride
(CaCl.sub.2) or other salt which will dissolve in water. Metal
layer 16 can be shadow masked and have an interior region 18 which
is non-solder wettable metal (such as titanium) and an outer ring
19 of a solder wettable (such as platinum) metal. Solder bumps 20
are then applied to the wettable metal 19. The wettable metal 19
and solder bump 20 may take generally the same geometric form as
shown in FIG. 4 so that when the device of FIG. 7 is flipped and
applied to the device shown in FIG. 3, solder bumps 20 will overlay
the metal pads 14.
FIG. 8 shows the two halves, FIGS. 3 and 7, joined together. The
solder is then reflowed securing the diaphragm 16 tight against
stand-offs 15. The soluble substrate is then dissolved as shown in
FIG. 9 and the solder is reflowed again to stretch the diaphragm.
After substrate 17 is dissolved, diaphragm 16 has two exposed
surfaces, one facing lower field plate 12 and one facing outwardly.
The diaphragm now forms a capacitive acoustic transducer between
itself and layer 12. Acoustic energy impinges upon the outwardly
facing surface of field plate 16. First field plate 12 and second
field plate 16 form a capacitor which will respond to this acoustic
energy for providing a variable output. Contacts to first field
plate 12 and to second field plate 16 can then be made for
connecting the capacitive transducer to the electronic circuitry
which may be integrated into substrate 11.
FIGS. 10 and 11 show the transducer in more detail. FIG. 10 shows a
top view which includes substrate 11 having first field plate 12
suitably formed therein and second field plate 16 suitably mounted
thereon. First contact 30 is provided within substrate 11 for
making contact with first field plate 12 and second contact 31 is
formed for providing electrical contact to second field plate
16.
As shown in FIG. 11, first field plate 12 is suitably formed as by
way of diffusion in semiconductor substrate 11. Substrate 11 is
then coated with a silicon dioxide layer 33 as a result of normal
semiconductor processing techniques. A hole is etched in layer 33
so that metallic layer 34 can be deposited over silicon dioxide
layer 33 and make contact with first field plate 12 at contact
point 30. Metallic layer 35 is also deposited over silicon dioxide
layer 33. Passivation layer 13 is next formed over the metal and
silicon dioxide layers as shown with a hole etched therein so that
contact can be made at contact point 31 to metal layer 35. Pads 14
are then placed on the substrate structure as shown in FIG. 2.
Stand-offs 15 are also formed as shown in FIG. 3.
Next, the second field plate structure is formed as shown in FIGS.
5-7, flipped, and joined to the first field plate structure. The
soluble salt is washed away, the solder reflowed and the diaphragm
stretched.
Stand-offs 15 may be considered to be part of the contact means
which may include pads 14, solder bumps 20 and wettable metal pads
19. Stand-off rings 15 may not be necessary if contact pads 14 and
19 and solder bumps 20 provide enough support for diaphragm 16.
Metal leads 34 and 35 now provide the leads from the respective
field plates 12 and 16 of the capacitive transducer.
Any variation in sound waves impinging upon diaphragm 16 will cause
the capacitance between field plates 12 and 16 to vary changing the
output signal on the leads 34 and 35.
* * * * *